Throughout the development of the electronics industry, lead-based solder, primarily tin-lead (Sn—Pb) has been a mainstay material for reliably joining electronic components. Due to its relatively low melting temperature and electrical conductivity, it is well suited for this purpose. However, growing awareness of the toxicity of lead to humans has led to numerous controls on its usage, primarily to prevent it from entering the environment through the waste stream when electronic devices are discarded. Some nations and economic entities have instituted schedules for the mandatory removal of lead from many products, banning import and sale of products containing lead within the near future.
The electronics industry has scrambled to find suitable replacement materials for the lead solder in their products. ‘Lead-free’ replacement solders, such as SnAg(Cu) are typically stronger than traditional Sn—Pb solders, but they also have a significantly higher melting (reflow) temperature (approximately 230-270° C.) than Sn—Pb (approximately 183° C.). These two properties, combined with other technological developments, have exacerbated the incidence of thermal stress damage in electronic components. During such processes as die attach, thermal stresses develop in the silicon die during solder reflow, frequently damaging 125 the mechanically fragile low k interlayer dielectric 105 (ILD) materials used in current silicon devices. Damage such as cohesive failure 115 (e.g. cracking) within a solder joint or solder ball 104, or adhesive failure 120 (e.g. delamination) between a solder ball 104 and a pad 103 of either a substrate 102 or a semiconductor device 101, may also be more common problems when using lead-free solders. Next generation ILD materials are expected to be even more fragile, and problems from thermally induced stress damage are expected to become even more pronounced.
Thermal fatigue is also a significant reliability concern. During manufacturing and normal use, solder materials and other materials within electronic devices are subjected to thermal cycling. When the coefficients of thermal expansion (CTE) of solder and other materials in a device are mismatched, solder joints may be subjected to inelastic strain accumulation, thermal fatigue, and eventually, damage such as fatigue crack growth.
With deadlines approaching for providing lead-free products to markets requiring them, the electronics industry continues to try to develop reliable, lead-free solder materials that are less susceptible to many failure mechanisms observed with current lead free solder materials, as well as some lead-containing solders.